Technology for Room-Temperature Hard Anodizing of Aluminum Alloy in Mixed Electrolyte Consisting of Sulfuric Acid-Oxalic Acid-Tartaric Acid

Al alloy (2024-T3) was hard anodized in the mixed electrolyte consisting of sulfuric acid as the major component and oxalic acid plus tartaric acid as the minor components. The effects of power-supply modes, electrolyte components and anodizing conditions (temperature, current density and oxidation time) on the hardness and thickness of the hard anodizing coatings were investigated. The microstructure and elemental composition of the hard anodizing coatings were analyzed by means of scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. It was found that the optimal hard anodizing electrolyte consisted of 10% (volume fraction) sulfuric acid as well as 30 g/L oxalic acid and 30 g/L tartaric acid. The optimal anodizing conditions were suggested as constant direct current overlapped pulse power, current density of 2 A/dm2, temperature of 15 ℃ and oxidation time of 50 min. Resultant hard anodizing coating had a thickness of 50 μm and hardness of 350 HV.